Once you have find the group you are looking to make the reservtion for using the Group Search function or if you see the group on the dashboard screen just click on any reservation. 

For easy identification groups are shown in Italics. 

Once you select a reservation you will be in the Group Bookings area 

On the right-hand side of this panel, you will see two options, Add a Room and Action. 

Once you click on Add a Room you will see a booking screen similar to below. Just fill out the details for the new reservation and click Create. The new reservation will be added to the group.